This invention relates generally to affinity purification of proteins and more specifically to the use of a modified bis-arsenical fluorescein compound immobilized to a solid support for protein purification.
Many techniques in the biological sciences require attachment of labels to molecules, such as polypeptides. For example, the location of a polypeptide within a cell can be determined by attaching a fluorescent label to the polypeptide.
Traditionally, labeling has been accomplished by chemical modification of purified polypeptides. For example, the procedures for fluorescent labeling require that the polypeptide be covalently reacted in vitro with a fluorescent dye, then repurified to remove excess dye and/or any damaged polypeptide. Using this approach, problems of labeling stoichiometry and disruption of biological activity are often encountered. Furthermore, to study a chemically modified polypeptide within a cell, microinjection can be required. This can be tedious and cannot be performed on a large population of cells.
Thiol- and amine-reactive chemical labels exist and can be used to label polypeptides within a living cell. However, these chemical labels are promiscuous. Such labels cannot specifically react with a particular cysteine or lysine of a particular polypeptide within a living cell that has numerous other reactive thiol and amine groups.
A more recent method of intracellular labelling of polypeptides in living cells has involved genetically engineering fusion polypeptides that include green fluorescent protein (GFP) and a polypeptide of interest. However, GFP is limited in versatility because it cannot reversibly label the polypeptide. The ability to generate a wide range of specifically labeled molecules easily and reliably would be particularly useful.
The use of genetically-encoded affinity tags is now a standard method of purifying proteins (reviewed in (Hannig and Makrides, 1998; LaVallie and McCoy, 1995; Makrides, 1996; Nilsson, et al., 1997 ; Uhlxc3xa9n and Moks, 1990)). This technique allows for simple purification of a protein of interest by fusing to it a tag with affinity for a stationary phase. Most affinity tags are small-molecule binding proteins (e.g. maltose binding protein, glutathione S-transferase). However, the size of these proteins can potentially interfere with the protein to which they are fused. A few short peptides, which are potentially less perturbing, have been used as affinity tags. The most common ones are the 6xc3x97histidine tag and the FLAG tag (a 6 amino acid antibody epitope). However, both these affinity tags have disadvantages. The FLAG tag requires the use of an expensive antibody affinity matrix and as a result has not received widespread use. The polyhistidine tag, which binds to metal ions, is very widely used, but requires somewhat harsh conditions (either high concentrations of imidazole or low pH) for elution, which can disrupt macromolecular complexes. In addition, small amounts of metal ions that elute with the protein can inactivate many enzymes. The purity of the eluted protein can be low because many histidine-rich proteins can bind to and elute from metal affinity resins, contaminating the purified protein.
Recently, a fluorescent dye has been developed which specifically interacts with tetracysteine containing helices (Griffin, et al., Science 281:269-272 (1998)). This compound, known as FlAsH (fluorescein arsenical helix binder), has been shown to specifically interact with proteins tagged with a C-C-X1-X2-C-C containing helix. The interaction is readily reversed by incubation with small dithiols such as ethanedithiol. FlAsH affinity chromatography is a highly specific protein purification method. It is based on the regiospecific interaction of two arsenics in FlAsH with two pairs of cysteines in a target alpha helix. The only requirement for binding is that the protein of interest contain the motif C-C-X1-X2-C-C within an alpha helix. This motif is rarely found in proteins, and labeling tagged proteins in vivo (Griffin, et al., Science 281:269-272 (1998)) indicate that there are very few eukaryotic proteins that bind to FlAsH. In contrast, many organisms contain histidine-rich proteins, and binding of these proteins to metal ion resins is a major source of contamination.
SUMMARY OF THE INVENTION
The present invention is based on the discovery of a method for protein purification using a modified fluorescein arsenical helix binder (FlAsH) compound, immobilized to a solid support, which can yield substantially pure protein from a single purification step.
A particularly preferred modified bis-arsenical molecule of the invention has the following formula: 
The tautomers, anhydrides and salts of the modified bis-arsenical molecule of formula (I) are also included.
Preferably, the modified bis-arsenical molecule specifically reacts with a target sequence, within the bonding partner, to generate a detectable signal, for example, a fluorescent signal.
The modified bis-arsenical molecule preferably is capable of traversing a biological membrane. The modified bis-arsenical molecule preferably includes a detectable group, for example a fluorescent group, luminescent group, phosphorescent group, spin label, photosensitizer, photocleavable moiety, chelating center, heavy atom, radioactive isotope, isotope detectable by nuclear magnetic resonance, paramagnetic atom, and combinations thereof.
In a first aspect, the invention includes a method for isolating a polypeptide of interest including contacting a modified Fluorescein Arsenical Helix (FlAsH) binder compound immobilized on a solid support with a sample containing a polypeptide of interest under conditions that allow binding of the polypeptide to the immobilized FlAsH compound, and eluting the polypeptide of interest, which has been modified by the addition of the FlAsH target sequence motif C-C-X1-X2-C-C (SEQ ID NO: 1,), where X1 and X2 are any amino acid, from the immobilized FlAsH compound. The polypeptide of interest which contains the FlAsH target sequence is the bonding partner of the modified FlAsH compound. xe2x80x9cBonding partnerxe2x80x9d as used herein refers to a molecule that contains at least the target sequence.
Another aspect of the invention provides a DNA construct containing an origin of replication, a selectable marker, a promoter that allows expression of the polypeptide of interest, and a cloning site, wherein the 5xe2x80x2 end of the cloning site contains a genetically-encoded affinity tag, and wherein the 3xe2x80x2 end of the cloning site contains a FlAsH target sequence motif.
Yet another aspect of the invention provides a DNA construct comprising an origin of replication, a selectable marker, a promoter that allows expression of the polypeptide of interest, and a cloning site, wherein the 5xe2x80x2 end of the cloning site contains a FlAsH target sequence motif, and wherein the 3xe2x80x2 end of the cloning site contains a genetically-encoded affinity tag.
Still another aspect of the invention provides a method for producing a polypeptide of interest which contains at its N-terminus a genetically-encoded affinity tag and at its C-terminus contains a FlAsH target sequence motif including, expressing a DNA sequence which encodes the polypeptide of interest as in the above-mentioned DNA construct, in an appropriate cell type, and producing the polypeptide of interest in an appropriate cell type.
Another aspect of the invention provides a method for isolating a polypeptide of interest, wherein the polypeptide contains at its N-terminus a genetically encoded affinity tag and at its C-terminus a FlAsH sequence motif. The method includes contacting a sample which contains a polypeptide of interest with an affinity resin which binds to the affinity tag; eluting the polypeptides bound to the affinity resin; contacting a modified FlAsH compound immobilized on a solid support with the polypeptides eluted from the first affinity resin, under conditions that allow binding of the polypeptide to the FlAsH compound; and eluting the polypeptide of interest from the immobilized FlAsH compound.
Still another aspect of the invention provides a method for isolating a polypeptide of interest, wherein the polypeptide contains at its N-terminus a genetically-encoded affinity tag and at its C-terminus a FlAsH sequence motif including: contacting a sample which contains a polypeptide of interest with a FlAsH compound immobilized to a solid support; eluting the polypeptides bound to the immobilized FlAsH compound; contacting an affinity resin with the polypeptides eluted from the FlAsH affinity resin, under conditions that allow binding of the polypeptide to the second affinity resin; and eluting the polypeptide of interest from the second affinity resin.
Another aspect of the invention provides a method for producing a polypeptide of interest containing at its N-terminus a FlAsH target sequence motif and at its C-terminus a genetically-encoded affinity tag. The method includes expressing a DNA sequence which encodes a polypeptide of interest from the DNA constructs of the present invention which allow addition of either the FlAsH target sequence motif or another genetically-encoded affinity tag, in an appropriate cell type, and producing the polypeptide of interest, modified to contain affinity tags, in the appropriate cell type.
Yet another aspect of the invention provides a method for isolating a polypeptide of interest, containing at its N-terminus a FlAsH target sequence motif and at its C-terminus a genetically-encoded affinity tag. The method includes contacting a sample which contains a polypeptide of interest with an affinity resin which binds to the affinity tag; eluting the polypeptides bound to the affinity column; contacting a modified FlAsH compound immobilized on a solid support with the polypeptides eluted from the affinity resin, under conditions that allow binding of the polypeptide to the FlAsH compound; and eluting the polypeptide of interest from the immobilized FlAsH compound.
Another aspect of the invention provides a method for isolating a polypeptide of interest, which has at its N-terminus a FlAsH target sequence motif and at its C-terminus a genetically-encoded affinity tag. The method includes contacting a sample which contains a polypeptide of interest with a FlAsH compound immobilized to a solid support; eluting the polypeptides bound to the immobilized FlAsH compound; contacting an affinity resin with the polypeptide eluted from the immobilized FlAsH compound, under conditions that allow binding of the polypeptide to the affinity resin; and eluting the polypeptide of interest from the affinity resin.
In yet another aspect, the invention features a kit that includes all the materials and reagents to carry out the invention as described for both FlAsH affinity purification or dual affinity tag purification. A kit of the invention includes at least a modified FlAsH compound as described herein.